Many familiar electronic devices or controls therefor operate on power that must be provided at a substantially constant voltage. such a substantially constant voltage must generally be derived from alternating current (AC) power distribution systems, often referred to as the grid, where power is provided as a periodically varying voltage for efficiency of power transmission over substantial distances, or batteries which provide power at a voltage which varies more-or-less slowly with the state of charge of the battery or the amount of potential energy stored in their constituent materials. Therefore, a power converter must be used to derive the substantially constant voltage required and many different types of power converters, sometimes referred to as voltage regulators have been developed of both analog and switching types. In general, switching type power converters are generally preferred at the present time since they are much more efficient than analog regulators which develop a voltage drop and thus consume power that must be dissipated as heat when current is drawn through them.
Many different topologies of switching power converters have been developed in recent years and a number of different control techniques have also been developed. Among known control techniques, current mode control is often favored because of the simple loop compensation, good current sharing and current limiting it provides. However, when the duty cycle of a switching power converter is above 0.5 for peak current mode control (or below 0.5 for valley current mode control) subharmonic oscillations occur due to the double pole at one-half the switching frequency in control-to-inductor current transfer function that is moved to the right half-plane. Therefore, it is conventional to include an externally developed ramp function in the feedback loop to assist in stabilizing the system. Conventionally, a fixed external ramp slope, se is chosen based on compensation for the worst-case duty cycle and component tolerance change or variation and thus such a slope will cause over compensation under normal conditions; reducing current loop strength and weakening the benefit of current mode control. The quality factor of the double pole is also reduced under normal conditions and becomes very low, causing significant drop in gain and phase drop in the transfer function which, in turn reduces system bandwidth. To minimize the impact of the fixed ramp slope based on worst case conditions, several ramp strategies have been proposed.
All known strategies for adaptively altering ramp slope to avoid overcompensation require accurate a priori knowledge of the Ri/L ratio to achieve good tracking performance. However, these values can easily vary over a range of 80% to 150% of a nominal value since variation includes changes to component value tolerance, temperature change and bias condition of real components. Further, the percentage tracking error is proportional to real Ri/L tolerance and the tracking error, in turn, causes wide quality factor variation which results in a significant phase error and double pole peaking in the control-to-inductor current transfer function. The peaking in the transfer function also limits the bandwidth of the system.
Another complicating factor at the present time is the fact that a different analysis and compensation of the external ramp slope is required for different power converter topologies in combination with the fact that power converters that can operate with different topologies are currently favored for many applications. For example, a so-called buck-boost converter operates as a buck converter when battery voltage is above a required voltage and operates as a boost converter when battery voltage is below required voltage so that battery utility can be extended.